1. Field of the Invention
The present invention relates to the field of fluid separation. More specifically, the present invention relates to the separation of oil and water in connection with hydrocarbon production activities.
2. Background of the Invention
Effective separation of water from produced crude oil is a continuing need for the oil industry. Effective separation is particularly useful during the early stages of production from a well when there may be high water content. Even in wells that do not have significant initial water production, water cuts can increase over the life of a well to the point where the production fluids have to be treated to remove water.
When water is produced with oil it is frequently in the form of an emulsion. An emulsion is a heterogeneous liquid system involving two immiscible liquids, with one of the liquids being intimately dispersed in the form of droplets in the second liquid. The matrix of an emulsion is called the external or continuous phase, while the portion of the emulsion that is in the form of small droplets is called the internal, dispersed, or discontinuous phase.
The stability of an emulsion is generally controlled by the type and amount of surface-active agents present. In some instances, particularly with heavy oils, finely divided mineral solids existing within the production stream can act as emulsifying agents. The emulsifying agents form interfacial films around the droplets of the dispersed phase and create a barrier that slows down or inhibits coalescence of the water droplets.
The tendency of heavy oils to contain water-in-oil emulsions is attributable to the presence of certain hydrocarbon molecules sometimes found in heavy crudes. Particularly, high naphthenic acid and asphaltene content crude oils possess the tendency to form stable, water-in-crude oil emulsions. The polar naphthenic acids and asphaltenes in crude oil stabilize dispersed water droplets. Further, sub-micron sized solids like silica and clay, when present in the crude oil, interact with the polar acids and asphaltenes to enhance the stability of the emulsions. Formation of stable water-in-crude emulsions results in difficulty in separating water from the crude oil.
For bitumen produced from oil sands, both water and solids result from the oil sands extraction process. This means that solids are also separated from the crude oil. Crude oil dehydration treating systems are typically used to reduce the basic sediment and water (BS&W) out of crude oil to the acceptable level specified by a crude oil purchaser, such as a pipeline company. The level of sediment and water typically specified by purchasers is less than 1%.
It has been known to separate water from crude oil using settling tanks and mechanical separators. However, when water forms a stable emulsion with crude oil, the use of storage or settling tanks and mechanical separators may fail to provide the separation desired. Emulsions of heavy oil and water produced from a reservoir formation can contain from about 1% to about 60% water by volume. A common range of emulsified water in crude oils heavier than 20° API is from 10% to 35%.
In an effort to further separate produced water from crude oil, it is also known to treat the well stream (i.e. the production fluids) with chemicals. These chemicals are referred to as dehydration chemicals or demulsifiers. Demulsifiers allow the dispersed droplets of the emulsion to coalesce into larger drops and settle out of the matrix. For example, U.S. Pat. No. 5,045,212; U.S. Pat. No. 4,686,066; and U.S. Pat. No. 4,160,742 disclose examples of chemical demulsifiers used for breaking emulsions. In addition, commercially available chemical demulsifiers, such as ethoxylated-propoxylated phenolformaldehyde resins and ethoxylated-propoxylated alcohols, are known for demulsification of crude oils. Such demulsifiers further minimize the amount of heat and settling time otherwise required for separation. However, the effectiveness of these demulsifiers on heavy crude oils, particularly those containing asphaltenes, naphthenic acids and inorganic solids may be limited.
Where the crude oil is heavy oil, it is typical to also employ electrostatic separators. Gravity settling and centrifugation in conjunction with chemical demulsifiers have also been employed.
It is also a known practice to increase the temperature of operation of separators in an attempt to break water/oil emulsions. U.S. Pat. No. 4,938,876 (herein referred to as the '876 patent) discloses a method for separating oil, water and solids from emulsions by heating the emulsion to about 115° C., rapidly cooling the mixture to below 100° C., separating the solids from the liquids and then separating the water from the oil. The '876 patent describes applying “an effective amount of a surfactant as a demulsifying agent” before heating. The patent further discloses the addition of a flocculant prior to cooling the mixture.
In some known technologies for breaking emulsions, an intermediate emulsion rag layer is produced. Further processing of the rag layer may be utilized to recover the crude oil and discharge the water. Recently, a microwave technology has been disclosed in U.S. Pat. Nos. 6,086,830 and 6,077,400. This microwave technology uses microwaves to treat hard-to-treat emulsions, especially for the rag layer. Other fluid treatment processes have been in U.S. Pat. No. 6,189,613 and U.S. Pat. No. 6,491,824.
There remains a need for improved demulsification processes for oil/water emulsions, such as heavy crude oil emulsions and bitumen emulsions. There is also a need for an improved fluid separation process in which a flocculant is applied to the well stream, followed by a demulsification and separation process. A need also exists for improved demulsification of heavy crude oils stabilized by solids-crude oil polar complexes.